1. Field of the Invention
The present invention relates generally to transmitted data monitoring apparatus, and more particularly, to a high speed data monitor capable of displaying a particular segment of data selected from a transmitted block of data.
2. Description of the Prior Art
Digital data transmission via data modems usually is in the form of fixed-length blocks of data which also contain a synchronization code and various other information such as source codes, destination codes, serial numbers, error codes, etc., in addition to the actual data. If the input data for a particular data block is insufficient to fill that data block, such as where, for example, the input data rate is lower than the transmission rate, filler data of a defined pattern is inserted to complete the data block. Should an error occur in the transmission, it is likely that the receiving equipment will not be able to interpret the data and perform the intended operation. For example, the transmission source, normally a computer, may make errors which would cause a misinterpretation of the received data; a programming error may result in the generation of erroneous identification codes; a telephone line connection error may result in informaation being addressed to the wrong terminal; or faults may occur in the transmitting or receiving equipment. In any case, when the receiving terminal equipment fails to process the received data, the cause for the failure must be determined.
Heretofore, in order to locate and identify data errors, either special computer programs were used to print the received data, or a general purpose oscilloscope was used to display the received bit pattern. However, the use of special computer programs is extremely impractical since considerable cost is involved in writing the special programs, the execution of the special program prevents the execution of regular processing programs, and a dedicated computer must be allocated for diagnosing the suspected data transmission problem. Moreover, where the received data rate exceeds the speed of the computer printer not all data can be printed, the interpretation of large volumes of printed data is difficult and time consuming, and where the problem is caused by the computer itself, the computer's program or interfacing equipment, no data printout will be obtained.
Another problem is that no capability is usually provided for monitoring the received data stream simultaneously with the normal processing of the data received.
Furthermore, the use of a oscilloscope as a means of inspecting the transmitted data is severely limited by the requirements for a external trigger source synchronous with the received data block. Although certain types of receiving equipment are capable of generating a suitable trigger signal, even in those devices the delayed sweep feature of the oscilloscope will have to be adjusted by trial and error to bring the desired data segment into view. Perhaps the most important disadvantage of using the oscilloscope method is that it is nearly impossible to assure that the data segment being viewed on the oscilloscope is in fact the data segment of interest.